From DE 198 59 129 A1 a cooling system for a supercharged internal combustion is known, which comprises a charge air cooling circuit and a refrigerant circuit. A liquid low-temperature coolant circulates in the charge air cooling circuit. The charge air cooling circuit has a low-temperature charge air cooler for cooling the charge air, and a low-temperature coolant cooler for cooling the low-temperature coolant. On the other hand, a refrigerant circulates in the refrigerant circuit. The refrigerant circuit has a vaporiser for vaporising the refrigerant, and a condenser for condensing the refrigerant. In the known cooling system the vaporiser is coupled fluidically with the charge air cooling circuit, so that the heat required for the vaporising of the refrigerant is removed from the low-temperature coolant. Hereby, the cooling efficiency of the charge air cooling circuit can be improved.
A similar cooling system is known from U.S. Pat. No. 6,006,540. There, the vaporiser is arranged in a storage container for the storage of the low-temperature coolant, wherein this storage container is in turn arranged in a branch of the charge air cooling circuit, which is connected in parallel to a branch of the charge air cooling circuit containing the low-temperature coolant cooler. By means of corresponding valve means, the coolant flow in the charge air cooling circuit can be divided, depending on requirements, to the low-temperature coolant cooler and the storage container. Therefore, here also additional cooling efficiency can be realized in the charge air cooling circuit by coupling with the refrigerant circuit.
Through the use of a charging device, for example an exhaust gas turbocharger, the supercharging, i.e. the pressure increase of the charge air, takes place, which leads to an increase in efficiency of the internal combustion engine. The compression is necessarily accompanied here by an increase in temperature. By cooling the charge air, the density of the air can be increased and therefore the air mass flow can be increased, which can be supplied to combustion chambers of the internal combustion engine. At the same time, improved pollutant emission values can be thereby realized. In particular, through cooling, the tendency to the formation of nitrogen oxides is reduced. Through the supercharging of the charge air, the internal combustion engine can therefore realize a comparatively high efficiency or respectively high loads. For these upper charge states, which are designated below as “full load”, it generally applies that the more intensive the cooling of the charge air, the better for the efficiency and the pollutant emissions of the internal combustion engine.
However, internal combustion engines in vehicles are not operated permanently in these upper load states, i.e. at full load. Rather, in fact in road traffic there are a variety of situations in which the internal combustion engine only has to be operated with low load or even only with basic load, so-called “idle mode”, for example, in typical stop-and-go situations and in the case of stopping at traffic lights. In these operating states of the internal combustion engine in the lower load range, which is designated in simplified form below as “partial load”, it has been found that an intensive cooling of the charge air is counterproductive with regard to the pollutant emissions of the internal combustion engine and with regard to the overall energy efficiency of the internal combustion engine. For example, the active cooling of the charge air requires energy which must be produced by the internal combustion engine. Furthermore, a reduced compatibility with respect to an exhaust gas recirculation can occur, when the charge air is too cold at partial load.
Accordingly, the need exists to cool the charge air only depending on requirements. Ideally, the charge air is therefore only to be cooled at full load, whereas at partial load it is supplied virtually uncooled to the combustion chambers of the internal combustion engine. Additional advantages thereby result for the partial load operation. Since with uncooled charge air with a greater volume flow a smaller air mass flows to the combustion chambers, a throttle valve of a fresh air system, which serves for controlling the air mass supplied to the combustion chambers, can be opened further, so that as a whole the throttle losses on the fresh air side can be reduced.
A problem here is that in the operation of the internal combustion engine, in particular in the case of a use in a motor vehicle, the various operating states, i.e. partial load and full load, can follow one another very rapidly. For reasons of comfort, it is required here that a transient state, which defines the transition from the partial load to the full load, is as short as possible. The internal combustion engine is to react as immediately as possible to an increased efficiency requirement. In supercharged internal combustion engines which operate with an exhaust gas turbocharger, in fact the so-called “turbo lag” can occur here. The response time of the turbocharger can be extremely improved by costly measures within the exhaust gas turbocharger, such as for example by a variable turbine geometry and/or by a waste gate valve. In particular, a modern turbocharger therefore requires less time than a conventional cooling system, in order to be transferred from the partial load into the full load. Therefore, the necessary high charge pressure is indeed available in an acceptable time, however, the compressed and thereby heated charge air can not be cooled sufficiently rapidly so that the required efficiency of the internal combustion can not yet be provided. This is only possible when the charge air cooling can also develop its full efficiency.
The need therefore exists to provide for a supercharged internal combustion engine a cooling system which has an as short as possible transient state for the transition from partial load to full load, in order to be able to realize a sufficient cooling of the charge air as rapidly as possible.